50 THE SURVIVAL OP THE UNLIKE. [l. 



My attention was first called to this line of thought 

 by contemplating upon the fact that cultivated plants 

 differ widely in variability, and I was struck by the fact 

 that many of our most inextricably variable groups — as 

 the cucurbits, maize, citrus, and the great tribes of 

 composites — are still unknown in a fossil state, presum- 

 ably because of their recent origin. Many other varia- 

 ble genera, to be sure, are well represented in fossil spe- 

 cies, as roses (although these are as late as the Eocene), 

 pyrus, prunus, and musa; but absolute age is not so 

 significant as the comparative age of the type, for types 

 which originated very far back may be yet in the com- 

 parative youth of their development. The summary 

 conclusions of a discussion of this subject were pre- 

 sented to the American Association for the Advance- 

 ment of Science two years ago.* A modification of 

 these points, as I now understand them, would run 

 something as follows: 



1. There is a wide difference in variability in culti- 

 vated plants. Home species vary enormousl}^ and oth- 

 ers but little, 



2. This variability is not correlated with age of culti- 

 vation, degree of cultivation, nor geographical distribu- 

 tion. 



3. Variability of cultivated plants must be largely 

 influenced and directed, therefore, by some antecedent 

 causes. 



4. The chief antecedent factor in directing this vari- 

 ability is probably the age^of the type. New types, in 

 geologic time, are polymoi'iihous; old types arc mono- 



♦See Proc. A. A. A. S., 1894; Botaiiicj.) (iii/.cttc. XIX., 3SI. 



